The overall form of a hydraulic excavator has been well known for decades. There are many examples of such excavators with a 2-boom stick hydraulically and rotationally secured to work within in a vertical plane, itself rotatable about a vertical axis coincident with a engine/cab combination mounted to a pair of tracks. The position and movement of the distal working end of the stick is controlled typically from a operator's cab mounted on the excavator. Although this general design profile is common, the range of sizes and shapes varies considerably due to the type and extent of work to be conducted.
The most common working tool used with a hydraulic excavator, among many, is a digging bucket rotatably secured to the distal end of the stick for rotation about a horizontal axis. The bucket is independently hydraulically driven in rotation about the horizontal axis by a cylinder positioned to apply hydraulic force between the stick and the bucket. The power of such machines and the amount of energy involved has and continues to increase with increasingly larger, heavier and more difficult materials and includes not only digging but also breaking and shearing, among others. Generally the bucket cylinder and its related hydraulic lines and connections are positioned on the outside of the stick well away from the working parts of the bucket and its various motions. The outside is that part of the operating range which lies outside of the work area between the stick/bucket and an engine/cab.
Necessarily, hydraulic excavators are designed to carry out a broad range of construction and demolition duties in extremely adverse and highly variable conditions, including high impact loading during initial contacts and continuing work, abrasive debris and severe vibration while working in all manner of solid, semi-solid and liquidy or mixed materials. Any tool failure or work failure is not only unacceptable but also very dangerous and must occur under tightly controlled conditions. Typically the operator is or prefers to concentrating on the machine interaction with the work area and not exclusively the moving parts of the excavator other than the actual working tool. As such, operational conditions, with or without error or failure, and with or without high energy events.
The variety of work types and conditions is exacerbated by location of the work which is often far beyond the reach of support or repair opportunities and even, on occasion, far beyond immediate accident support.
In many cases the work being done or which could be done would be facilitated by expanded flexibility in the form and use of the excavator as a whole and by an increase in the speed and continuity of operations. Thus, modern excavators are available with multiple working tools and with quick-connect mounting components adapted for rapid and operator-free changing of at least the primary working tool.
While convenient, these requirements bring about mechanical complexity as the bucket or other primary working tool is no longer a permanent fixture at the distal end of the stick. Consequently it is advantageous to secure as many moving parts, particularly relatively fragile and expensive hydraulic parts against the harsh environment in which they are called to operate.
To increase utility, in the past traditional bucket arrangements have been coupled with additional support tools such as thumb-like arrangements which operate to secure materials within and against the body of the bucket. Of these, there have been several common types, including:                1. a fixed retractable thumb secured to the stick,        2. a hydraulic thumb secured between the stick and the bucket for rotation about a single horizontal axis on the stick. The bucket and thumb are in relative rotation and motion during working and carriage of a load.        3. a stick pivot thumb where the thumb rotates on the same pivot point as the original bucket pin, not a secondary weld-on pivot point. A hydraulic cylinder is also directly connected between the thumb and the stick, providing rotation in relation to the bucket. The thumb and the bucket pivot on the same axis providing for maintenance of a constant grip on the load with suitable hydraulic circuits. The pivot eliminates scraping and slippage and reduces risk of release during rotation, reducing complexity, and,        4. more recently a hydraulic thumb secured to the bucket for rotation about a horizontal bucket axis driven by a hydraulic cylinder operating between the thumb and the sick.        